A line start motor is an electric motor that is equipped with both permanent magnets and short circuit bars in its rotor. A conventional line start permanent magnet motor has magnets inside the induction starting cage. It is known from the prior art that permanent magnet motors have good operating characteristics at synchronous speed. Typical rotors for line start permanent magnet motors are made of a plurality of laminated rotor sheets stacked on the top of each other in order to form a core. Punched slots are provided in the rotor sheets and these slots are configured to receive an aluminum die cast squirrel cage and magnets. A set of thin laminated rotor sheet are required in order to avoid generation of eddy currents in the axial direction of the rotor. The squirrel cage is used to start up and run the motor approximately up to synchronous speed. At synchronous speed the magnets take over and keep the rotor rotating at the synchronous speed.
The bridges between an air filled magnet slot and the adjacent aluminum slot is of great importance. Those bridges must be thick enough to give the required mechanical strength and to ensure that no aluminum flows from the aluminum slot to the magnet slot, during the die cast process, because this will make it impossible to mount the magnets into the magnet slot.
The bridges should be kept as thin as possible seen from an electromagnetic point of view because a significant part of the flux from the magnet flows in the bridges and does not link with the stator current. The problem is that this leakage magnet flux does not contribute to the torque. Accordingly, the bridges must be kept as thin as possible in order to until is as much of the magnets as possible.
US 2010/0133941 A1 discloses a rotor for a line start permanent magnet motor.
In this patent application the bridges are greater than 1 mm and thus a significant magnetic flux leakage is to be expected.
It is well known that each magnet slot for the permanent magnets in a line start motor has to be surrounded by a magnet free end area. This end area is needed in order to avoid short circuit of the magnetic flux so that a greater part of the end portion of the permanent magnets can be used to generate torque. WO 2008/137709 A2 discloses a rotor for a line start permanent magnet motor in which the bridges are bar shaped. This geometry is used very frequently in the prior art.
WO 0106624 A1, on the other hand, a rotor for a line start permanent magnet motor in which the magnet free end area has a basically triangular shape.
In the prior art the bridges are greater than 1 mm and thus a significant magnetic flux leakage is to be expected.